This invention relates to automotive brake master cylinder vacuum booster devices and, more particularly, to improvements in those of the type comprising a booster shell, an axially slidable booster piston accommodated in the booster shell, a first working chamber defined in the booster shell by the booster piston and held in communication with a source of vacuum supply, a second working chamber defined in the booster shell by the booster piston and selectively placed in communication with the first working chamber or the external atmosphere through a control valve means, and an input rod arranged opposite to the booster piston for movement toward and away from the latter and connected to the control valve means so as to produce a pressure difference between both the working chambers effective to cause the booster piston to follow forward movement of the input rod. Conventionally, when above type of the booster device is installed in an automobile, rear wall of the booster is fixed to the automobile the body while the master cylinder is supported on the front booster wall, so that forward thrust loaded on the master cylinder under the oil pressure inside the master cylinder pressure chamber produced by means of the booster device is transmitted to the automobile body through the booster shell. Accordingly, under the necessity of sufficient rigidity for bearing the thrust loading, the booster shell has hitherto been made from steel plates, aluminum alloys, etc. of sizable thickness causing a problem of considerable increase in weight thereof.
For reducing the weight of the booster shell, it has recently been proposed that the thrust loading be transmitted to the automobile body through a tie rod extending through the booster piston for connection of front and rear walls of the booster shell. To this end, a flexible bellows may be used for effectively sealing a tie rod through hole in the booster piston without preventing operation of the booster piston. The bellows is conventionally of very weak flexural rigidity and the middle portion thereof has a tendency to be weighed down and touch the upper side of the tie rod, so that the durability of the bellows may be decreased by rubbing the tie rod during repeated expansions and contractions of the bellows.